1. Field of the Invention
The present invention relates in general to a process for fabricating multi-level (multiple data states) read-only memory (ROM) devices. In particular, the present invention relates to a process for fabricating multi-level ROM devices that can be quickly delivered to a customer who places an order and that are substantially compatible with ROM devices produced by conventional fabrication processes.
2. Description of Related Art
Semiconductor memory devices have become indispensable components for modern digital electronic products. As fabrication techniques evolve, higher and higher integration densities are achieved. The devices become smaller in physical size while having increased memory capacity and data access speed. Read-only memory (ROM) devices are quite popular because of their non-volatile data storage characteristic. They have become a mainstay of digital electronic systems such as mini computer and microprocessor-based computer systems for storing basic level fixed program codes which are seldom altered.
ROM devices, however, are relatively difficult to make and require many complicated fabrication process steps. Many of these fabrication steps require time-consuming material handling and precise control of process conditions.
All ROM devices of the same model have the same basic structural configuration. The difference from ROM device to ROM device of a given model is caused by the different code contents held in their memory cells. For semiconductor fabrication factories, a vast number of ROM devices of the same model can be fabricated in substantially the same process up to the step at which specific cells are programmed. These "almost finished" devices are sometimes referred to as "unfinished" or "half" products. They can be safely stock piled to await final programming in response to a customer order.
Once a customer issues the order for a particular model of ROM device to hold a special code content, the stocked ROM half products can be quickly programmed and become ready for shipment in a relatively fast and simple programming phase. Programming of these ROM devices is normally carried out by producing a programming photomask that actually programs the code content of the memory cells in each of these devices. Such post-programming procedure has become the standard practice for most mask ROM semiconductor device manufacturers.
Mask ROM having buried bit lines is one of the most popular mask ROM semiconductor memory devices in use today. This is primarily because this type of mask ROM has a relatively small unit area per memory cell. This characteristic can be translated into higher data storage density per device. To facilitate the fabrication of such high-density ROM devices, several fabrication processes involving the concept of multi-level data storage per memory cell were developed. Among these processes, three are worth mentioning.
In one known process ions are implanted at different doses to control the threshold voltage of various cells so as to achieve multi-level data storage. This approach has several drawbacks. More than one photomask is required for multiple ion implantations. It is time-consuming to have to align each of the multiple photomasks, apply photoresists, ion implant with different controlling conditions, remove photoresists, and withdraw the photomasks. Also, desired threshold voltages are not easily controllable.
A second known process fabricates gate oxide layers having different thicknesses for controlling memory cell current to achieve multi-level data storage. This approach also has its limitations. This kind of process requires that ROM device fabrication can only begin after the code content to be programmed has been decided. This takes considerable lead time for product delivery which is unacceptable for many customers. In this process, as with the previously described one, multiple photomasks are required to fabricate oxide layers of different thickness. Finally, the thickness of oxide layers is difficult to control.
A third known process manipulates channel width to achieve multi-level data storage. This approach also has its problems. More than one photomask is necessary. The post-programming process is also relatively more time-consuming than acceptable. And, for most current ROM device production line fabrication facilities, it is not yet possible to implement such a process in a smooth and relatively simple way.